Air classification of flour

ABSTRACT

Air-classification of flours with a particle size of 5-160 μm is best performed on flours that are remilled at least once to give a remilled flour with an x-50 of less than 40 μm. 
     The remilling, preferably being carried out by using a jet air mill, a Retsch mill or an extruder. In this way a higher yield of a fraction, enriched in protein (up to 40% protein) and a higher yield of a starch fraction with a low protein content (even less than 3 wt %) can be obtained. Wetting of the remilled flour, prior to remilling and drying of the remilled flour, prior to air classification lead to improved results.

FIELD OF THE INVENTION

The present invention concerns a method for the air classification offlour having a particle size distribution of 5-160 μm.

DESCRIPTION OF PRIOR ART

Although air classification of such flours is known in the prior art(cf. Jones et al, J. Biochem. Microbiol. Techn. Eng. 1 (1959), 77-97;Stringfellow et al, Northwest Miller 270 (1964), 19), there are still anumber of problems attached to these prior art methods. Thus, forexample, the yields of protein-enriched product fractions wererelatively low, whereas the protein contents of the starch-enrichedfractions were too high for them to be useful as a starch. This is a.o.due to the fact that Stringfellow et al. first apply air classificationand then remilling of the fines obtained. A second air classificationcan then be applied to the remilled fines. Jones et al. disclose thatpremilling, prior to air classification can lead to fines with anincreased protein content. However, only small amounts of fines with theincreased protein content were obtained (cf. table V). Jones furtherapplies a process, wherein the water content of the flour prior tore-milling is decreased (cf. Table V).

The above-mentioned problems are probably related with insufficientdisrupture of the endosperm particles. Therefore, we have studiedmethods by which this disrupture could be improved. As a result of thesstudies, we have found that remilling of the flour under specificconditions can improve this disrupture, resulting in high yields offines having an increased protein content and in starch fractions havinga very low protein content.

SUMMARY OF THE INVENTION

Therefore, our invention is concerned with a method for the airclassification of flour having a particle size distribution of 5-160 μm,wherein:

a flour with an enhanced water content is remilled at least once, togive a remilled flour having a specific size diameter X-50 of less than40 μm, preferably less than 30 μm, in particular 15-25 μm.

the resulting, remilled flour is air-classified, applying an air flowrate setting and/or a classifier speed with which a predetermined cutpoint can be achieved.

collecting a protein-enriched fraction as a fine fraction in a yield ofat least 30 wt. %, and a starch-enriched fraction as a coarse fractionin a yield of at most 70 wt. %.

DETAILED DESCRIPTION OF INVENTION

The specific size diameter X-50 is defined as the diameter whereby 50%of the flour particles have a diameter of less than X-50 (onvolume/volume base).

The cut point is defined as that particle size at which all particlesbelow the cut point are in the fine fraction and all particles above thecut point are in the coarse fraction.

The air classification can be performed with any type of air classifier,e.g. an Alpine Lab Zig-Zag-100 MZR classifier®. The cut point can bechosen by selecting a specific air flow rate setting and/or classifierspeed.

Particle size measurements were performed, using a Sympatec HelosParticle Sizer® equipped with a 100 mm lens. All particle sizemeasurements were carried out on water suspensions, which were stirredmagnetically and sonificated.

The purpose of the remilling is to disrupt the endosperm of the particlein a more efficient way. We have found that this purpose is achievedwhen the remilling leads to a specific size diameter X-50 of less than40 μm, preferably less than 30 μm, in particular 15-25 μm. These valuesfor X-50 are obtained when the remilling is performed at least once, butpreferably twice, using a jet air mill, a Retsch mill® or an extruder.Although the best results are obtained with a jet air mill, we prefer touse the Retsch mill since the energy input is very high for a jet airmill. The amount of damaged starch in the remilled flour is in generalless than 8 wt %. However, this is dependent on the hardness of theflour and the mill applied for the remilling.

The air classification is carried out with standard equipment. Theefficiency of the air classification is determined mainly by the cutpoint selected. In general, a cut point of 20 or less is applied.

The two fractions which are separated by air classification are:

1) a fine fraction having a particle size below the cut point,containing 18-40 wt. % of protein, in particular 20-30 wt. % of protein,most preferably 25-30 wt % of protein.

2) a coarse fraction having a particle size above the cut point,containing at most 7 wt. %, preferably less than 5 wt. % and inparticular less than 3 wt. % of protein.

This latter fraction therefore consists mainly of starch. Starchfractions having very low protein contents are very useful for severalapplications, whereby the presence of protein should be avoided as muchas possible.

The exact protein content of the two fractions will greatly depend onmany factors, such as protein content of the starting flour, efficiencyof the starch/protein disruptures in the endosperm, type of remiller,cut point etc.

It is, however, preferred to use flours having a protein content of10-15 wt. % as starting material. In particular, soft flours having arelatively high protein content are preferred. Examples of flours thatcan be used include Zeeuws®, Camp Remy® (both low in protein: 8-9 wt.%), Monopol®, CWRS® and DNS® (11-13 wt. % of protein). However, manyother types of flours exist that can be used.

We have found, unexpectedly, that the efficiency of the remilling of theflour is influenced by the moisture content of the flour subjected toremilling. In fact, we have found that flour having a relatively highmoisture content gave the best results when subjected to airclassification after remilling.

Therefore, we apply a flour with an enhanced water content. We prefer toincrease the moisture content of the flour subjected to remilling, priorto remilling, by 10-30% of the original moisture content of the flour.

However, the best results with air classification are obtained when theremilled flour is dried prior to air classification. Therefore, weprefer to decrease the water content of the remilled flour, prior to airclassification, by 10-50% of the original moisture content of theremilled flour.

Accordingly, the best sequence of process steps is:

selecting a flour having a particle size of 5-160 μm and a moisturecontent of 10-15 wt. %;

increasing the moisture content by 10-30 wt. %;

remilling the moistened flour to specific size diameter X-50<40 μm;

drying the remilled flour, reducing the moisture content by 10-50 wt. %;

air-classifying the dried, remilled flour, using a cut point of at most20 μm;

collecting a fraction of fine particles (≧30%) and a fraction of coarseparticles (≦70%).

Parts of the invention are also (1) flour fractions having a specificsize diameter of less than 20 μm and a protein content of 18-40 wt. %,preferably 20-30 wt % and (2) starch fractions having a specific sizediameter of more than 20 μm and a protein content of less than 3 wt. %.

Doughs prepared from flour consisting at least in part of theabove-mentioned protein-enriched flour fractions are also part of theinvention. Further, the use of the starch fraction according to theinvention in sponge-doughs or in batters is part of the invention.

Experimental Part

In this experimental part, the flours having the composition mentionedin Table I were used.

The protein content was determined by ICC Standard 136 (=Kjeldahl).

The SDS sediment was determined by suspending a sample in a 1.4%solution of SDS protein (ICC Standard 116).

The moisture content was determined by drying at 130° C. for 1.5 hours(ICC Standard 110/1).

                  TABLE I                                                         ______________________________________                                        COMPOSITION OF WHEAT FLOURS                                                             Protein Ash        Moisture                                                                             SDS                                                 content content    content                                                                              sediment                                  Sample    (%)     (%)        (%)    (mm)                                      ______________________________________                                        Zeeuws     8.8    0.51       14.5   40                                        Camp       8.7    0.47       14.0   92                                        Remy                                                                          Monopol   11.2    0.43       14.2   95                                        CWRS      13.3    0.45       14.0   92                                        DNS       13.3    0.45       15.8   95                                        ______________________________________                                    

Air Classification

In order to establish the cut point or vane setting for optimalseparation of wheat flours into separate fractions, i.e. small starchkernels plus protein bodies and intact flour particles plus large starchkernels, we have investigated, with the aid of air classification, anuntreated commercial soft wheat flour (Zeeuws) and a hard wheat flour(Monopol) at different cut points. The yield, protein content andprotein shift of the fine and coarse fractions were determined. Theresults are summarised in Tables II and III. These results show that inboth cases the highest protein content was obtained with a cut point of10 μm. The fine fraction of Zeeuws flour had a protein content of 19.8%and accounted for 6.2% of the flour weight and 14% of the total proteincontent.

The fine fraction of Monopol, on the contrary, had a protein content of18.8% and accounted for 4.2% of the flour weight and 6% of the totalprotein content.

The bigger the cut points, the higher the yield of the fine fraction;however, the lower the protein content, the lower the effect ofseparation. This means: the bigger the cut points, the slighter theseparation between protein bodies and intact flour particles, thushardly any protein shift.

The results also show that a soft flour had a higher yield of theprotein bodies containing the fine fraction than a hard flour.

A soft flour contains more disaggregated flour particles. Therefore, asoft flour will respond better to air classification than a hard wheatflour.

After a second classification of the fine fraction obtained at a cutpoint of 20 μm, a cut point of 10 μm gave a fine fraction having aprotein content of 23% and 26.7% of protein, respectively, for Zeeuwsand Monopol. In this case, more protein shifted into the high proteinfraction but nevertheless it was 13 and 6%, respectively, of the totalprotein. This means therefore the same as that obtained at a cut pointof 20 μm.

                  TABLE II                                                        ______________________________________                                        AIR CLASSIFICATION OF FLOUR                                                   (Zeeuws)                                                                                                     Protein                                                                              Percentage                                                      Yield  content                                                                              of total                                Cut point     Fractions (%)    (%)    protein                                 ______________________________________                                        Flour: 14.5% moisture and 8.8% protein                                                      fines      6.2   19.8   14                                      10 μm                                                                                    coarses   91.7   8.2    86                                                    fines     13.4   13.8   20                                      20 μm                                                                                    coarses   83.7   8.3    78                                                    fines     26.9   9.6    30                                      30 μm                                                                                    coarses   71.5   8.6    70                                                    fines     43.5   9.3    46                                      40 μm                                                                                    coarses   55.3   8.7    54                                      Fraction: fines of 20 μm                                                                 fines     38.5   23.0   13                                      10 μm                                                                                    coarses   60.0   7.6     7                                      ______________________________________                                    

                  TABLE III                                                       ______________________________________                                        AIR CLASSIFICATION OF FLOUR                                                   (Monopol)                                                                                                    Protein                                                                              Percentage                                                      Yield  content                                                                              of total                                Cut point     Fractions (%)    (%)    protein                                 ______________________________________                                        Flour: 14.2% moisture and 11.2% protein                                                     fines      4.2   18.8    6                                      10 μm                                                                                    coarses   94.7   12.3   94                                                    fines      8.5   18.1   12                                      20 μm                                                                                    coarses   92.7   12.0   88                                                    fines     14.5   14.2   17                                      30 μm                                                                                    coarses   84.2   12.2   83                                                    fines     22.4   12.8   24                                      40 μm                                                                                    coarses   76.4   12.2   76                                      Fraction: fines of 20 μm                                                                 fines     34.5   26.7    6                                      10 μm                                                                                    coarses   61.0   13.9    6                                      ______________________________________                                    

Effect if Remilling

The above-mentioned results have shown that only a small part of acommercial flour consists of disaggregated flour particles. In order toobtain higher yields of fine, protein-enriched fractions, remilling ofthe flour is necessary.

Different mills were tested, viz. Jet air mill, Roller mill, Alpine pinnmill and a Retsch mill equipped with a 0.8 mm screen.

The remilled flours obtained with different mills were air-classified ata cut point of 20 μm. The results are summarised in Table IV.

The highest yield of the fine fraction was obtained with the flourremilled with a Jet air mill.

The lowest yield was obtained with the flour remilled with a Rollermill.

The Retsch mill also gave acceptable results.

Table V shows the results obtained with Zeeuws flour that had beenremilled once. From the results it can be concluded that airclassification at a cut point of 20 μm gave the highest yield ofprotein-enriched fraction. The fraction has a protein content of 15% andaccounted for 37% of the flour weight and 66% of the total proteincontent.

Tables VI and VII show the results obtained by air classification offour flours which were remilled once or twice. Zeeuws flour gave thehighest yield of the fine fraction, viz. 41.1%. In all cases there isclearly an additional protein shift to the fine fraction after a secondremilling action.

The lowest protein content of the coase fraction was obtained withZeeuws flour.

                  TABLE IV                                                        ______________________________________                                        AIR CLASSIFICATION OF REMILLED FLOURS                                         OBTAINED WITH DIFFERENT MILLS                                                 Flour:      DNS, 15.8% moisture and 13.3% protein                             Air classification:                                                                       cut point 20 μm                                                                             Protein                                                                              Percentage                                Type of             Yield    content                                                                              of total                                  mill      Fractions (%)      (%)    protein                                   ______________________________________                                         Jet air      fines     36.1   25.2   70                                      mill          coarses   63.2    6.1   30                                       Roller       fines     22.4   19.0   32                                      mill          coarses   76.1   12.0   68                                       Alpine       fines     25.5   21.2   40                                      mill          coarses   74.1   11.2   60                                       Retsch       fines     32.2   20.4   49                                      mill          coarses   68.1   10.2   51                                      ______________________________________                                    

                  TABLE V                                                         ______________________________________                                        AIR CLASSIFICATION OF FLOUR (Zeeuws)                                          Effect of remilling with a Retsch mill                                        Flour: Zeeuws, remilled 1x                                                                                   Protein                                                                              Percentage                                                      Yield  content                                                                              of total                                Cut point     Fractions (%)    (%)    protein                                 ______________________________________                                                      fines     30.2   17.2   60                                      10 μm                                                                                    coarses   68.4   5.0    40                                                    fines     37.1   15.0   66                                      20 μm                                                                                    coarses   60.4   5.1    34                                                    fines     49.3   10.1   56                                      25 μm                                                                                    coarses   50.3   7.5    43                                                    fines     52.6   9.2    54                                      30 μm                                                                                    coarses   47.2   8.4    46                                      ______________________________________                                    

                  TABLE VI                                                        ______________________________________                                        AIR CLASSIFICATION OF FLOURS                                                  Effect of remilling once and twice                                            Air classification: cut point 20 μm                                                                Protein Percentage                                                 Yield      content of total                                      Sample       (%)        (%)     protein                                       ______________________________________                                        DNS (original)          13.3                                                  *fines        5.1       19.2     7                                            *coarses     94.2       13.1    93                                            remilled 1x                                                                   *fines       32.2       20.4    49                                            *coarses     68.1       10.2    51                                            remilled 2x                                                                   *fines       36.6       27.1    74                                            *coarses     62.5        5.5    26                                            CWRS (original)         13.3                                                  *fines        3.2       20.2     5                                            *coarses     96.2       13.2    95                                            remilled 1x                                                                   *fines       26.1       18.0    35                                            *coarses     73.1       12.5    65                                            remilled 2x                                                                   *fines       31.2       25.1    57                                            *coarses     68.1        8.3    43                                            ______________________________________                                    

                  TABLE VII                                                       ______________________________________                                        AIR CLASSIFICATION OF FLOURS                                                  Effect of remilling once and twice                                            Air classification: cut point 20 μm                                                                Protein Percentage                                                 Yield      content of total                                      Sample       (%)        (%)     protein                                       ______________________________________                                        Camp Remy                8.7                                                  *fines        7.1       12.0     9                                            *coarses     92.5        8.4    91                                            remilled 1x                                                                   *fines       23.9       17.2    48                                            *coarses     75.3        6.0    52                                            remilled 2x                                                                   *fines       30.2       18.2    63                                            *coarses     70.1        4.5    37                                            Zeeuws (original)        8.8                                                  *fines       13.2       13.8    22                                            *coarses     83.7        8.3    78                                            remilled 1x                                                                   *fines       37.1       15.0    66                                            *coarses     60.4        5.1    34                                            remilled 2x                                                                   *fines       41.1       17.1    80                                            *coarses     57.2        3.2    20                                            ______________________________________                                    

Effect of moisture content on flour

Zeeuws flour having a moisture content of 15% was moistened to 17% andtwo other batches were dried to 12 to 10% moisture, respectively. Theseflours were remilled with the aid of a Retsch mill. First of all, theparticle size distribution of these flours was determined.

Table VIII shows the X-50 values of the remilled flours. Afterremilling, the flour having a moisture content of 17% had an X-50 of 20μm. The lower the moisture content, the higher the X-50 value, whichmeans the lower the disaggregation of the flour particles. These flourswere also air-classified at a cut point of 20 μm. The results are shownin Table IX. The higher degree of disaggregation of flour particles inthe flour sample having 17% moisture content finds also expression inthe air classification.

We have also investigated the effect of moisture content on theefficiency of air classification.

Remilled Zeeuws flour having a moisture content of 14% was dried to amoisture content of 8%. Both flours were air-classified. The resultsshown in Table X illustrate that drying of the flour has a positiveeffect on the separation into fines and coarses by air classification.

                  TABLE VIII                                                      ______________________________________                                        PARTICLE SIZE DISTRIBUTION OF REMILLED                                        FLOURS EFFECT OF MOISTURE                                                                      Specific particle size                                       Sample           diameter: X-50                                               ______________________________________                                        Zeeuws, 17.0% moisture                                                                         20 μm                                                     Zeeuws, 14.8% moisture                                                                         23 μm                                                     Zeeuws, 12.0% moisture                                                                         26 μm                                                     Zeeuws, 10.0% moisture                                                                         30 μm                                                     ______________________________________                                    

                  TABLE IX                                                        ______________________________________                                        AIR CLASSIFICATION OF FLOURS                                                  MENTIONED IN TABLE VII                                                               Flour:     Zeeuws                                                             Air classification:                                                                      cut point 20 μm                                                                 Protein Percentage                                                Yield       content of total                                       Sample     (%)         (%)     protein                                        ______________________________________                                        Flour, 17.0%                                                                  moisture                                                                      *fines     36.1        18.1    73                                             *coarses   62.4         3.8    27                                             Flour, 14.8%                                                                  moisture                                                                      *fines     37.1        15.0    64                                             *coarses   60.2         5.4    36                                             Flour, 12.0%                                                                  moisture                                                                      *fines     30.1        14.2    49                                             *coarses   68.2         6.5    51                                             Flour, 10.0%                                                                  moisture                                                                      *fines     26.1        13.1    37                                             *coarses   73.1         8.0    63                                             ______________________________________                                    

                  TABLE X                                                         ______________________________________                                        AIR CLASSIFICATION OF FLOURS HAVING A                                         DIFFERENT MOISTURE CONTENT                                                           Flour:     Zeeuws                                                             Air classification:                                                                      cut point 20 μm                                                                  Protein Percentage                                                 Yield      content of total                                      Sample       (%)        (%)     protein                                       ______________________________________                                        Remilled flour                                                                moisture content                                                              14%                                                                           *fines       37.1       15.0    64                                            *coarses     60.1        5.4    36                                            Remilled flour                                                                moisture content                                                              8%                                                                            *fines       38.2       18.1    72                                            *coarses     60.1        4.5    28                                            ______________________________________                                    

I claim:
 1. A method for the air classification of flour having aparticle size distribution of 5-160 μm, comprisingenhancing the watercontent by 10-30% of the original moisture content of the flourremilling the flour of enhanced water content at least once, to give aremilled flour having a specific size diameter X-50 of less than 40 μm,air classifying the resulting remilled flour by applying the air flowrate setting and a classifier speed with which a predetermined cut pointcan be achieved, to form a protein enriched fraction and astarch-enriched fraction and collecting the protein-enriched fraction asa fine fraction in a yield of at least 30 wt %, and the starch-enrichedfraction as a coarse fraction in a yield of at most 70 wt. %.
 2. Amethod according to claim 1, wherein the remilling is carried out in amilling system chosen from the group consisting of a jet air mill, aRetsch mill or an extruder.
 3. A method according to claim 2, whereinthe remilling is carried out to give flour with X-50=15-25 μm.
 4. Amethod according to claim 1, wherein the remilling is carried out twice.5. A method according to claim 1, wherein flour having a protein contentof 10-15 wt. % is used.
 6. A method according to claim 1, wherein a cutpoint of 20 or less is used.
 7. A method according to claim 1, whereinthe fraction containing the fine particles contains 18-40 wt. % ofproteins.
 8. A method according to claim 1, wherein the fractioncontaining the coarse particles contains at most 7 wt. % of proteins. 9.A method according to claim 1, wherein the moisture content of theremilled flour, prior to air classification, is reduced by 10-50% of theoriginal moisture content of the remilled flour.